The chemical kinetics and product analysis of thermal and photolytic degradation of methotrexate (MTX) in aqueous solution was studied. Major degradation products were identified using simple column chromatography and HPLC. These studies led to a mechanism of decomposition of MTX in aqueous solutions. The stability of various commercial dosage forms and one I.V. infusion solution of MTX was studied and found to be acceptable within the recommended storage periods. The impurities commonly present in commercial MTX were found not to be due to degradation products, but were from the synthesis of MTX itself.